1. Field of the Invention
The invention relates to a liquid crystal color picture screen provided with a light source, a polarizer, a liquid crystal cell comprising two transparent plates between which a liquid crystal material is present, said plates each supporting a matrix of electrodes, a front plate, and a photoluminescent layer.
2. Description of the Prior Art
Liquid crystal picture screens are based on the principle that light from an external light source either does or does not pass through the layer of liquid crystals. The resulting bright and dark dots constitute a picture. The use of color filters also renders it possible to manufacture liquid crystal color picture screens. The use of color filters is not very efficient, because a major portion of the incident light is absorbed by these color filters. The combination of a polarizer-analyzer unit with color filters is also unfavorable, because a major portion of the incident light is converted into thermal energy. These effects result in a reduction of the brightness and of the energy efficiency of the liquid crystal color picture screen.
A further problem is the reduced viewing angle of liquid crystal picture screens, which results from the operating principle and construction of a liquid crystal picture screen.
A liquid crystal color picture screen is known from EP 0 889 350 A1 in which a photoluminescent layer having an isotropic absorption and a polarized chromatic emission is used instead of a conventional polarizer. The same document also describes a liquid crystal color picture screen in which a photoluminescent layer is provided between the liquid crystal cell and the viewer. A particularly favorable aspect of this arrangement is that the emission of the colored light is independent of viewing angle narrowing effects of the liquid crystal layer. The photoluminescent layer comprises, for example, a polyolefin film which is doped with a dichroic fluorophor, preferably poly(phenyleneethynylene) derivatives.
The fluorophors described in the patent application, however, usually have a very low emission quantum efficiency, a low dichroic ratio of parallel absorption to perpendicular absorption, or a short useful life.
The invention has for its object to provide an improved liquid crystal color picture screen with an improved viewing angle and an improved luminous efficacy.
This object is achieved by means of a liquid crystal color picture screen provided with a light source, a polarizer, a liquid crystal cell comprising two transparent plates between which a liquid crystal material is present, said plates each supporting a matrix of electrodes, a front plate, and a photoluminescent layer, which layer comprises a dichroically absorbing color agent and an isotropically emitting color agent.
The arrangement of the functional elements in different color agent molecules renders it possible to synthesize and use optimized materials for the respective functions. A photoluminescent layer which has a high absorption and a high emission quantum efficiency increases the efficiency of the entire liquid crystal color picture screen.
It may be advantageous that the dichroically absorbing color agent and the isotropically emitting color agent are interlinked with one another.
An intramolecular energy transfer from one molecule unit to another may be more effective than an intermolecular energy transfer between two molecules.
It is preferred in this embodiment that the dichroically absorbing color agent and the isotropically emitting color agent are mutually interlinked by means of one or several compounds chosen from the group comprising phenylene units, methylene units, difluoromethylene units, peptide units, norbornyl units, spiro compounds, pyrazine units, cyclohexyl units, cyclopentyl units, and cyclobutyl units.
These units do not interfere with the electronic structure of the dichroically absorbing color agent or the isotropically emitting color agent, neither do they have electronic interactions therewith.
It is particularly preferred that the dichroically absorbing color agent has a high extinction coefficient for the wavelength emitted by the light source.
It is furthermore preferred that the isotropically emitting color agent has a low extinction coefficient for the wavelength absorbed by the dichroically absorbing color agent.
The dichroically absorbing color agent is effectively and exclusively excited under these conditions.
It is furthermore advantageous that the isotropically emitting color agent has a high extinction coefficient for the wavelength emitted by the dichroically absorbing color agent.
An effective energy transfer between the dichroically absorbing color agent and the isotropically emitting color agent is safeguarded under these conditions.
It is furthermore preferred that the isotropically emitting color agent comprises a perylene derivative, a laser color agent derivative, or a stilbene derivative.
The xcfx80xcfx80* energy level of these molecules is occupied by an energy transfer from the absorber to the emitter. The xcfx80xcfx80* energy level lies a few nanometers bathochromically shifted towards the dichroic absorption of the absorber. The emission of the colored light then takes place at wavelengths of 400 nm and higher. All colors can be created through variation of the isotropically emitting color agent.
It is furthermore advantageous that the dichroically absorbing color agent and the isotropically emitting color agent are embedded in a liquid crystal medium or in an elongate polymer, or are epitaxially grown on an anisotropic substrate.
The color agent molecules of the photoluminescent layer are arranged on a carrier in a preferred direction such that their longitudinal axes are oriented in the same direction. It is true in principle that the photoluminescent layer and the liquid crystal cells should be arranged as closely together as possible so as to prevent parallaxes.
It is furthermore preferred that the photolurninescent layer comprises a plurality of parallel strips, a pixel-type arrangement, or a zonal arrangement.
The strips comprise alternately a photoluminescent layer with dichroically absorbing color agent and a color agent which emits isotropically in one of the three basic colors. A pixel-type arrangement is thus obtained in combination with the grid-type electrodes.
In a preferred embodiment, the liquid crystal color picture screen comprises a color filter between the front plate and the photoluminescent layer.
Some isotropically emitting color agents such as, for example, perylene derivatives may be excited by incident ambient light. The contrast of the liquid crystal color picture screen may be impaired by this excitation. The use of a color filter can prevent this.
In a particularly preferred embodiment, the front plate comprises a UV mirror.
This UV mirror reflects non-absorbed UV radiation back into the optical system. On the one hand, this protects the viewer from this UV radiation, while on the other hand the luminous efficacy of the liquid crystal color picture screen is enhanced by this xe2x80x9crecyclingxe2x80x9d.